In recent years, there has been extensive development of projection apparatus that include: a polarization conversion device; a light-splitting optical system; optical path reflecting means; image display elements; and a projection optical system. The polarization conversion device provides uniform polarization of light from a white light source. The white light source can be: a short-arc metal halide lamp, in which a metal halogenide is sealed in a light-generating tube, the distance between the electrodes is kept short, and the light-emission properties of the metal are used to generate light; a super high pressure mercury lamp, which can easily provide high luminance; a xenon lamp, which provides superior color rendering properties; or the like. The light-splitting optical system separates light into red (R), green (G), and blue (B) components. The red, green, and blue lights are each associated with optical path reflecting means and an image display element. Light from these image display elements are modulated using a video signal, and the resulting light is guided to a color combining optical system formed by combining prisms. This color combining optical system performs superimposition, and the resulting color image is magnified and projected onto a screen by the projection optical system.
These devices require high-precision control of light polarization properties. The polarization beam splitter, which acts as a polarizer and light detector, determines the polarization properties, and therefore greatly affects the image quality of the projected image. For this reason, much research has been done in recent years on improving polarization beam splitter properties.
As an example, a polarization beam splitter includes: a multilayer dielectric film; and a member formed from a translucent material. The absolute value of the photoelastic constant of the translucent member is selected so that it is no more than a predetermined value associated with the wavelength of the incident light projected into the translucent member. Also, the polarized light separation module associated with red (R), green (G), and blue (B) light beams separate the light from the white light source into their associated colors. The base materials used in the optical members of these polarized light separation modules are designed to provide optimal characteristics by providing the lowest absolute value of the photoelastic constant at different wavelengths.
These polarized beam splitters are used as polarizers or light detectors. Various improvements have been made on illumination optical systems to provide efficient and uniform distribution of light beams from the light source. In one implementation, a television image or an image output from a computer is displayed on image display elements arranged in a matrix according to a predetermined vertical/horizontal aspect ratio, e.g., a liquid crystal panel, serving as means for modulating the intensity of the illumination light beam described above. The image displayed by the image display elements is displayed on a projection-type image projector apparatus that uses a projection lens to magnify the image or a “rear” type projection image display apparatus equipped with a screen on which to show magnified and projected images.
A screen aspect ratio of 4:3 (horizontal:vertical) is common for image display elements used to display computer screens. For displaying television images, the increase in digital cable broadcasts in the North American market and BS digital broadcasts in Japan has led to the growing popularity of an aspect ratio of 16:9 (horizontal:vertical) over the conventional 4:3 (horizontal:vertical) aspect ratio.